A normal ear directs sounds as shown in FIG. 1 from the outer ear pinna 101 through the generally cylindrical ear canal 110 (typically about 26 mm long and 7 mm in diameter) to vibrate the tympanic membrane 102 (eardrum). The tympanic membrane 102 moves the bones of the middle ear 103 (malleus, incus, and stapes) that vibrate the cochlea 104, which in turn functions as a transducer to generate electric pulses to the brain that are interpreted as sounds. In addition, the inner ear also includes a balance sensing vestibular system which involves the vestibular labyrinth 105, its three interconnected and mutually orthogonal semi-circular canals: the superior canal 106, posterior canal 107, and horizontal canal 108 (as well as the otolith organs, the utricle and saccule—not shown). The canals and spaces of the vestibular labyrinth 105 are filled with endolymph fluid which moves relative to head movements, thereby activating hair cells that send an electrical balance signal to the brain via the vestibular nerve 111.
In some people, the vestibular system is damaged or impaired, causing balance problems such as unsteadiness, vertigo and unsteady vision. Vestibular implants are currently under development, with one of the initial challenges being the relatively significant amount of power required by the gyroscope/accelerometer arrays used for the movement sensors (gyroscopes and linear accelerometers). Presently, one lower power device is the STMicroelectronics L3G4200D, which is a three-axis digital gyroscope that encodes all three-dimensional axes of rotation. This device measures 4 mm square by 1 mm thick and needs various power and signal lines to operate at the specified power requirement of at least 6 mA at 3.6V.
For use in an implant system, the power for one or more vestibular movement sensors can be supplied from a body worn battery pack and transcutaneously transmitted with a head placed coil. But the power losses for such a transcutaneous supply are roughly a factor of two and there also is an additional risk of the head coil falling off, power being lost, and the patient becoming disoriented and even falling. A somewhat better solution might be to have an implanted battery supplying power to the implanted movement sensors, but (due to the high power requirements) this approach is likely to require a large battery volume or very frequent re-charging of the battery. Furthermore, failure of any of the modules of the device would require re-implantation, with consequent risk to hearing and residual vestibular function.
Other arrangements have also been proposed for vestibular implant systems. For example, head-worn sensor arrays have been proposed that would be secured by a holding band around the head, but this approach would create an unacceptably high risk of movement of the sensors relative to the head. Implanted sensor arrays powered via a percutaneous plug have also been proposed, but the serious problems with percutaneous structures are notoriously well-known. Challa and Bhatti, A Micromachined Cupula: Toward Biomimetic Angular Velocity Sensor Prosthesis, 33rd Mid-Winter Research Meeting, Assn. for Research in Otolarygology, Feb. 6-10, 2010 (incorporated herein by reference) proposed a basic re-design of the sensor array around the fluidic principle used by the balance organ itself to reduce power requirements, but more time and extensive reliability testing will be needed to complete the development of such a device.
Application US2005/0267549 by Della Santina et al. (incorporated herein by reference) teaches a combined cochlea/vestibular stimulation system with a speech and motion sensing processor (SMP) placed either externally or internally. Application US2002/0104971 by Merfeld et al. (incorporated herein by reference) teaches a motion sensing system to be worn not only on the head but also on other body parts.
For safety reasons, it is important that the externally worn unit including the sensor is always placed in a known, correct orientation when driving the implant. Otherwise the sensor's misaligned input to signal processing, and ultimately to the neural stimulation sites, will lead to a mismatch between real and perceived head movement. Under specific circumstances this may cause a patient to fall and possibly result in injury. This is of special relevance when the implant is located on the head such that sometimes the patient cannot visually observe placing the external unit over the implant.
The correct placement of an external unit relative to an implant is currently solved for cochlear implants and other auditory implants by a pair of axially magnetized magnets. One magnet is placed in the center of the implant's receiver coil. The other magnet is placed in the center of the sender coil in the external unit. While placing the external unit's magnet in proximity to that of the implant, the magnetic attraction force causes the external coil to be placed over the implant's coil in a concentric orientation. But there is a remaining degree of freedom in that the external unit can be turned in the radial direction a full 360 degrees relative to the implant. Due to this degree of radial rotation freedom, this solution is not appropriate for placing an external sensor as part of a vestibular implant system.